Kepler-186f
(also known by its
Kepler object of interest designation ''KOI-571.05'') is an Earth-sized
exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...
orbiting within the
habitable zone
In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...
of the
red dwarf
A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...
star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
Kepler-186,
the outermost of five such planets discovered around the star by
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
's
Kepler space telescope. It is located about from
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
in the constellation of
Cygnus.
[ Distance to Kepler 186, after taking into account light extinction](_blank)
/ref>
Kepler-186f orbits its star at a distance of about from its host star with an orbital period of roughly 130 days
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, and night. This daily cyc ...
, and a mass and radius around 1.44 and 1.17 times that of Earth, respectively. As one of the more promising candidates for habitability, it was the first planet with a radius similar to Earth's to be discovered in the habitable zone of another star. However, key components still need to be found to determine its habitability for life, including an atmosphere
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
, and its composition and if liquid water can exist on its surface.
Analysis of three years of data was required to find its signal. NASA's Kepler space telescope detected it using the transit method (in which the dimming effect that a planet causes as it crosses in front of its star is measured), along with four additional planets orbiting much closer to the star (all modestly larger than Earth). The results were presented initially at a conference on 19 March 2014[ See session 19 March 2014 – Wednesday 11:50–12:10 – Thomas Barclay: The first Earth-sized habitable zone exoplanets.] and some details were reported in the media at the time. The planet was announced on 17 April 2014, simultaneously with publication of a scientific paper in ''Science
Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
''.
Physical characteristics
Mass, radius and temperature
The only physical property directly derivable from the observations (besides the orbit) is the size of the planet relative to the central star, which follows from the amount of occultation of stellar light during a transit. This ratio was measured to be 0.021, giving a planetary radius of 1.17 ± 0.08 times that of Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
. The planet is about 11% larger in radius than Earth (between 4.5% smaller and 26.5% larger), giving a volume about 1.37 times that of Earth (between 0.87 and 2.03 times as large).
A very wide range of possible mass
Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
es can be calculated by combining the radius
In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
with densities derived from the possible types of matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic pa ...
from which planets can be made. For example, it could be a rocky terrestrial planet
A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to ...
or a lower density ocean planet with a thick atmosphere. A massive hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
/helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
(H/He) atmosphere is thought to be unlikely in a planet with a radius below 1.5 . Planets with a radius of more than 1.5 times that of Earth tend to accumulate the thick atmospheres which make them less likely to be habitable. Red dwarfs emit a much stronger extreme ultraviolet (XUV) flux when young than later in life. The planet's primordial atmosphere would have been subjected to elevated photoevaporation during that period, which would probably have largely removed any H/He-rich envelope through hydrodynamic mass loss.
Mass estimates range from 0.32 for a pure water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
/ice composition to 3.77 if made up entirely of iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
(both implausible extremes). For a body with radius 1.11 , a composition similar to that of Earth (i.e., 1/3 iron, 2/3 silicate rock) yields a mass of 1.44 , taking into account the higher density due to the higher average pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
compared to Earth. That would make the force of gravity on the surface 17% higher than on Earth.
The estimated equilibrium temperature for Kepler-186f, which is the surface temperature without an atmosphere, is said to be around , somewhat colder than the equilibrium temperature of Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
.
Host star
The planet orbits Kepler-186, an M-type red dwarf
A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...
star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
which has a total of five known planets. The star has a mass of 0.54 and a radius of 0.52 . It has a temperature of 3755 K and is about 4 billion years old, about 600 million years younger than the Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, which is 4.6 billion years old and has a temperature of .
The star's apparent magnitude
Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...
, or how bright it appears from Earth's perspective, is 14.62. This is too dim to be seen with the naked eye, which can only see objects with a magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
up to at least 6.5 – 7 or lower.
Orbit
Kepler-186f orbits its star with about 5% of the Sun's luminosity with an orbital period of 129.9 days and an orbital radius of about 0.40 times that of Earth's (compared to for Mercury). The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from ). Kepler-186f receives about 32%, placing it within the conservative zone but near the outer edge, similar to the position of Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
in the Solar System.
Habitability
Kepler-186f's location within the habitable zone does not necessarily mean it is habitable; this is also dependent on its atmospheric characteristics, which are unknown. However, Kepler-186f is too distant for its atmosphere to be analyzed by the most advanced instruments such as the James Webb Space Telescope
The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...
. A simple climate model – in which the planet's inventory of volatiles is restricted to nitrogen, carbon dioxide and water, and clouds are not accounted for – suggests that the planet's surface temperature would be above if at least 0.5 to 5 bars of CO2 is present in its atmosphere, for assumed N2 partial pressures ranging from 10 bar to zero, respectively.
The star hosts four other planets discovered so far, although Kepler-186 b, c, d, and e (in order of increasing orbital radius), being too close to their star, are considered too hot to have liquid water. The four innermost planets are probably tidally locked, but Kepler-186f is in a higher orbit, where the star's tidal effects are much weaker, so the time could have been insufficient for its spin to slow down significantly. Because of the very slow evolution of red dwarfs, the age of the Kepler-186 system was poorly constrained, although it is likely to be greater than a few billion years. Recent results have placed the age at around 4 billion years. The chance that it is tidally locked is approximately 50%. Since it is closer to its star than Earth is to the Sun, it will probably rotate much more slowly than Earth; its day could be weeks or months long (see Tidal effects on rotation rate, axial tilt and orbit).
Kepler-186f's axial tilt
In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbita ...
(obliquity) is likely very small, in which case it would not have tilt-induced seasons like Earth's. Its orbit is probably close to circular, so it will also lack eccentricity-induced seasonal changes like those of Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
. However, the axial tilt could be larger (about 23 degrees) if another undetected non-transiting planet orbits between it and Kepler-186e; planetary formation simulations have shown that the presence of at least one additional planet in this region is likely. If such a planet exists, it cannot be much more massive than Earth as it would then cause orbital instabilities.
One review essay in 2015 concluded that Kepler-186f, along with the exoplanets Kepler-442b and Kepler-62f
Kepler-62f (also known by its Kepler Object of Interest designation ''KOI-701.04'') is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's Kep ...
, were likely the best candidates for being potentially habitable planets.
In June 2018, studies suggest that Kepler-186f may have seasons and a climate similar to those on Earth.
Follow-up studies
Target of SETI investigation
As part of the SETI Institute
The SETI Institute is a not-for-profit research organization incorporated in 1984 whose mission is to explore, understand, and explain the origin and nature of life in the universe, and to use this knowledge to inspire and guide present and futu ...
's search for extraterrestrial intelligence
The search for extraterrestrial intelligence (usually shortened as SETI) is an expression that refers to the diverse efforts and scientific projects intended to detect extraterrestrial signals, or any evidence of intelligent life beyond Earth.
...
, the Allen Telescope Array had listened for radio emissions from the Kepler-186 system for about a month as of 17 April 2014. No signals attributable to extraterrestrial technology were found in that interval; however, to be detectable, such transmissions, if radiated in all directions equally and thus not preferentially towards the Earth, would need to be at least 10 times as strong as those from Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
. Another search, undertaken at the crowdsourcing
Crowdsourcing involves a large group of dispersed participants contributing or producing goods or services—including ideas, votes, micro-tasks, and finances—for payment or as volunteers. Contemporary crowdsourcing often involves digit ...
project SETI-Live, reports inconclusive but optimistic-looking signs in the radio noise from the Allen Array observations. The more well known SETI @ Home search does not cover any object in the Kepler field of view.[See the fourth question at ] Another follow-up survey using the Green Bank Telescope
The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Rad ...
has not reviewed Kepler 186f. Given the interstellar distance of , the signals would have left the planet many years ago.
Future technology and observations
At approximately distant, Kepler-186f is too far and its star too faint for current telescopes or the next generation of planned telescopes to determine its mass or whether it has an atmosphere. However, the discovery of Kepler-186f demonstrates conclusively that there are other Earth-sized planets in habitable zones. The Kepler spacecraft focused on a single small region of the sky but next-generation planet-hunting space telescopes, such as TESS and CHEOPS, will examine nearby stars throughout the sky. Nearby stars with planets can then be studied by the James Webb Space Telescope
The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...
and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions. Additionally the Square Kilometer Array would significantly improve radio observations over the Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
and Green Bank Telescope
The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Rad ...
.
Previous names
As the Kepler telescope
The Kepler space telescope is a defunct space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
observational campaign proceeded, an initially identified system was entered in the Kepler Input Catalog (KIC), and then progressed as a candidate host of planets to a Kepler Object of Interest (KOI). Thus, Kepler-186 started as ''KIC 8120608'' and then was identified as ''KOI-571''. Kepler-186f was mentioned when known as KOI-571-05 or KOI-571.05 or using similar nomenclatures in 2013 in various discussions and publications before its full confirmation.[ See comment by "Holger 16 November 2013 at 14:21".]
^
^
^
Comparison
The nearest-to-Earth-size planet in a habitable zone previously known was Kepler-62f
Kepler-62f (also known by its Kepler Object of Interest designation ''KOI-701.04'') is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's Kep ...
with 1.4 Earth radii. Kepler-186f orbits an M-dwarf star, while Kepler-62f orbits a K-type star. A study of atmospheric evolution in Earth-size planets in habitable zones of G-Stars (a class containing the Sun, but not Kepler-186) suggested that 0.8–1.15 R🜨 is the size range for planets small enough to lose their initial accreted hydrogen envelope but large enough to retain an outgassed secondary atmosphere such as Earth's.
In popular culture
* Along with five other exoplanets, Kepler-186f was included in '' Civilization: Beyond Earth''s exoplanet DLC as a playable map.
* Dutch rock band named their 2017 album ''Kepler-186f'' after this exoplanet.[; ]
*Kepler-186f is the location of a future earth colony in the short story "Stars" by Drew Hayden Taylor
Drew Hayden Taylor (born 1 July 1962) is an Indigenous Canadian playwright, author and journalist.
Life and career
Born in Curve Lake, Ontario, Taylor is of both Ojibwe and white ancestry. About his background Taylor says: "I plan to start my ...
.
*In season 2 of the 2020 Animaniacs
''Animaniacs'' is an American Animated series, animated Comedy television, comedy Musical film, musical television series created by Tom Ruegger and produced by Warner Bros. Television Animation. It originally aired on Fox Broadcasting Company ...
reboot, Kepler 186f, alongside 51 Pegasi b
51 Pegasi b, officially named Dimidium (), is an extrasolar planet approximately away in the constellation of Pegasus. It was the first exoplanet to be discovered orbiting a main-sequence star, the Sun-like 51 Pegasi, and marked a breakthr ...
& the fictional WB-1, is referenced in the song "Yakko's Big Idea"
*In season 12 of The Big Bang Theory
''The Big Bang Theory'' is an American television sitcom created by Chuck Lorre and Bill Prady for CBS. It aired from September 24, 2007, to May 16, 2019, running for 12 seasons and 279 episodes.
The show originally centered on five charact ...
episode "The Conjugal Configuration", Kepler 186F is shown on a poster in Neil deGrasse Tyson's office.
See also
* Habitability of red dwarf systems
* List of potentially habitable exoplanets
* Lists of astronomical objects
This is a list of lists, grouped by type of astronomical object.
Solar System
* List of Solar System objects
* List of gravitationally rounded objects of the Solar System
* List of Solar System objects most distant from the Sun
* List of ...
References
External links
NASA – Mission overview
NASA – Kepler Discoveries – Summary Table
NASA – Kepler-186f
at The NASA Exoplanet Archive.
NASA – Kepler-186f
at The Exoplanet Data Explorer.
NASA – Kepler-186f
at The Extrasolar Planets Encyclopaedia.
Habitable Exolanets Catalog
at UPR-Arecibo.
NASA – Kepler 186f – SETI Institute – A Planet in the Habitable Zone (video)
2014.
NASA – NASA Press kit
.
{{Sky, 19, 54, 36.651, +, 43, 57, 18.06, 492
186f
Exoplanets discovered in 2014
Exoplanets in the habitable zone
Kepler-186
Transiting exoplanets
Near-Earth-sized exoplanets in the habitable zone
Cygnus (constellation)